A device for performing catalytic endothermic reactions is known from U.S. patent application, Ser. No. 386,942, filed Aug. 9, 1973, now U.S. Pat. No. 3,870,476, issued Mar. 11, 1975, FIGS. 4 through 8. This device comprises catalyst-filled tubes in which hydrocarbons are reformed in the presence of steam at elevated pressure. The heating gas side is also operated at elevated pressure to relieve mechanical stresses to which the tubes are exposed. The final temperature of the process gas generally exceeds 700.degree.C and is preferably higher than 800.degree.C. This hot process gas must be channelled through a multitude of tubes across the reaction vessel bottom head. The particular problem consists in providing for each reaction tube a separate feed and discharge line passing into and leaving the vessel that is pressurized by the heating gas while avoiding any undue enlargement of the vessel or any unjustifiable weakening of the head by a multitude of tube passage apertures. Separate feed and discharge lines are necessary to enable any reaction tube to be sealed off from outside in the event of damage.
The above patent application provides for feeding the hydrocarbons to be processed through individual flexible small size tubes that are capable of compensating thermal expansions. Because of the low feed stream temperature of approximately 500.degree.C the mechanical design at this point involves no problem. Conditions are totally different for the outlet. The high temperature prevailing on both the process gas and heating gas sides precludes for the reaction tubes a normal passage through the head with welding of the tubes into the head. Referring to the above patent application, the design provides for a passage through the head in that the hot gas from the reaction tube is transferred into a small size tube that is seal welded to the head through a cooling cap; the head is cooled by a cooling agent that circulates in a heat exchanger which is attached to the head direct, the cooling agent, for example water, being preferably held at a favorable pressure. In this way, the head separates three fluids in the area of the tube passage, viz. process gas, heating gas, and cooling agent.
This design is not entirely satisfactory in view of the presence of the small size outlet tube requiring additional critical welds and extending to both the heating gas side and water side. Owing to prevailing thermal conditions, this tube must be of a high alloy austenitic steel, while the risk of stress corrosion cracking exists for any austenitic steel on the water side.